1. Field of the Invention
The present invention relates to an image reading apparatus and an image forming apparatus, and more particularly, to an image reading apparatus that has a plurality of light sources and that is preferable in use for an image scanner, digital copier, facsimile, etc., and an image forming apparatus having the image reading apparatus.
2. Description of the Related Art
These years, an image reading apparatus capable of faster processing poses a problem of ensuring a fine SN (Signal-to-Noise ratio) of a read image. To ensure a fine SN, a light quantity at a manuscript (original document) surface must be increased. A conventional image reading apparatus having one light source, however, may be incapable of obtaining a desired light quantity because of limitations related to drive current, heat generation, etc. Hence an image reading apparatus equipped with a plurality of light sources has been suggested. An image forming apparatus having a plurality of light sources, however, shows a high possibility of the occurrence of light source abnormality, which raises a concern that a light source's being off due to the occurrence of abnormality leads to an inferior SN and the malfunction of an image processing circuit caused by the inferior SN.
Conventionally, such an abnormality as a light source's being off is detected by finding that the light source's output level is lower than a predetermined threshold level when the light source is activated. An image reading apparatus having one light source can detect a light source's being on and off relatively easily. An image forming apparatus having a plurality of light sources, on the other hand, has difficulty in detecting an abnormality due to a light source's being off or a decrease in light quantity.
A technique enabling detection of light source abnormality in an image reading apparatus having a plurality of light sources has been suggested. For example, Japanese Patent Application Laid-Open No. 2000-39679, etc., discloses such a known technique. According to this conventional technique, reference white levels are detected at a plurality of positions on a main scanning line to calculate the average of the reference white levels, and the abnormality of a light source is detected from the ratio of the white levels detected at a plurality of spots. This conventional technique, however, has a disadvantage that the abnormality cannot be detected from the ratio of white levels detected at the plurality of spots when the white level across the entire main scanning line deteriorates uniformly.
Another known conventional technique is described in Japanese Patent Application Laid-Open No. 2002-368955, etc. According to this conventional technique, the peak value of reference white levels on the main scanning line is detected and the light quantity of the light sources is adjusted so that the peak value becomes identical with a target value, then a light source is determined to be abnormal when the peak value cannot be adjusted to be identical with the target value. This conventional technique, however, accompanies the problem that abnormality detection takes much time, which makes detection for every manuscript reading operation impossible.
Offering a detection method utilizing the peak value of reference white levels, this conventional technique does not enable precise detection of emitter abnormality, because when an image reading apparatus has an illuminating system providing the synthesized light quantity of a plurality of light emitters as a manuscript surface light quality, the peak level on an end of the main scanning line hardly changes even if one of the light emitters working as light sources is not on.
As described above, according to the conventional technique, an image forming apparatus having a plurality of light sources has difficulty in detecting an abnormality due to a light source's being off or a decrease in light quantity, and is incapable of precise abnormal detection.